Non-circular cores are used in optical fibers and light guiding rods to optimize mode mixing. This can be used to homogenize beam profile or increase the coupling efficiency of pump light in active fibers. Due to the use of non-circular core shapes there are disadvantages too. Mode mixing leads to an increased loss especially of certain mode groups propagating mainly in the outer region of the core. This effect is highly relevant for core rods with a minimum of corner roundness. In addition to the increased loss there is a decrease of numerical aperture (NA). But in many applications it is very important to achieve a certain numerical aperture. The value of the NA can in principle be adjusted by the refractive index difference between the core and the cladding. For circular shaped cores it is possible to calculate the NA out of the refractive index difference. If an undoped core material is to be used and the cladding is doped only with fluorine it is possible to calculate the necessary fluorine concentration from the target refractive index difference and set this during preform production. Because of the additional losses based on mode mixing described before this simple approach does not work out for non-circular shaped core geometries.
There are some methods for the production on non-circular shaped core geometries known from the state of the art. In U.S. Pat. No. 4,859,223 a method is described producing a core section first and then removing the outer parts mechanically to obtain a non-circular core. Afterwards a cladding is applied and the preform drawn to a fiber. This results in desirable properties with respect to maintaining the polarization of certain modes.
U.S. Pat. No. 5,566,267 describes a production method for fibers with aspect ratios, which are aligned to the laser source. This results in a very high coupling efficiency. Furthermore an increased power density level can be obtained. The plasma based process used with this method deposits material mainly on the long side of the core and rarely at the edges.
German application DE 10 2009 004 756 describes a method where a rectangular core rod is jacketed with a cladding tube. This minimizes thermal stress and gives a better angularity. DE 10 2013 202 589 describes a method for the production of non-circular shaped core preforms with a thermal deposition of the cladding part. Due to the rounding of the corners an additional process step is necessary to straighten the edges mechanically.
U.S. Pat. No. 8,655,130 describes a method for the production of preforms with very small edge rounding. This is achieved by a modified plasma based inside deposition technique. So fibers with high angularity can be obtained, which results in coupling light energy out of the fiber core yielding in additional loss.
WO 14023799 describes a method for a highly homogenous dopand distribution in the cladding by depositing a first cladding layer to fill the valleys between the edges with a first set of process parameters. The preform is grinded to a round outer shape and prepared for the deposition of the second cladding layer. This layer is deposited under process parameters which might be identical to the first set, but do not have to be. This process is driven to have the dopand concentration of the second cladding layer identical with the first cladding layer.